This invention relates to improved methods and apparatus for the separation or beneficiation of particulate solid substances by means of an electrification mechanism generally classified under the heading "Electrostatic Separation", and more particularly to the separation or beneficiation of particulate materials containing a significant percentage of fines, i.e.: dust-like material ranging in size down to about 20 microns. The term "electrostatic separation" as used in this specification is intended to have the scope of meaning that is ascribed to it in "Chemical Engineers' Handbook", Robert H. Perry and Cecil H. Chilton, Editorial Directors; 5th Edition 1973, in the article entitled "Electrostatic Separation" at pages 21-62 to 21-65--McGraw-Hill Book Company, New York, New York. However, in the embodiments which are described in this specification, the invention is disclosed in relation to "high tension" separation methods and apparatus, which fall in Perry and Chilton's Group 3--Electrification by Ion Bombardment, described on pages 21-63 and 21-64 of their Handbook. Moreover, the apparatus illustrated in the accompanying drawings and described in the specification is the rotor type, in which particulate matter is delivered to a grounded rotor for separation or beneficiation.
High tension separation is an outgrowth of electrostatic separation, but has many unique properties of its own. The term "electrostatic" implies that no current is flowing. In high tension separation the particulate feed is sprayed with mobile ions, that is, a corona discharge, while the particles are being fed to and presumably come into contact with a grounded electrically conductive surface such as the surface of a rotating metal cylinder. In this way it is intended that all of the particles will be charged by the mobile ions, and that the particles of electrically non-conductive and poorly-conductive materials will lose their charges slowly, will be pinned to the grounded conductive surface by their own image forces, and will be removed from the grounded conductive surface at a location outside the influence of the corona discharge. The particles of electrically-conductive material, on the other hand, lose their charges rapidly to the grounded conductive surface and, upon being removed from the influence of the corona discharge (i.e.: the mobile-ion spray), they become free to assume normal trajectories away from the grounded electrical surface, under gravitational or centrifugal forces.
High tension electrostatic separation methods have worked well with, and have essentially been restricted to dry feeds in the size range of about 20 to about 150 mesh. An example of electrostatic separation as employed in the dry concentration of ion-bearing ores (e.g.: specular hematite) crushed to minus 20 mesh is described in U.S. Pat. No. 3,031,079. Pretreatment to provide discrete surfaces for selective electrification of individual particles has included dedusting and desliming (Perry and Chilton, ibid, at page 21-63). Examples given by the authors (at page 21-65) are: a minus 8-mesh grid would probably need disliming at 200 mesh; a minus 20-mesh grid at 325 mesh; and a minus 35-mesh grid at 400 mesh. As far as is now known to the present inventor, no successful application of electrostatic separation of dust-like materials has heretofore been made.
In the near-desperate attempts now being made to remove from coal sufficient of the sulfur content so that coal can be used as an energy source in place of oil, it has been found that pyrite is the major source of sulfur, and that pyrite can be distributed in various coals on a scale finer than 50 micrometers. It has also been found that coal which is pulverized that fine forms dense black clouds in a high tension separator, coating the electrodes and other parts, and the components of the coal-pyrite mixture cannot be separated. The long-felt want of an electrification mechanism for separating the components of a dust-like mixture of particles, which has been generally apparent in the art, is now seen to be a critical need of the nation's energy resources.